Referring to FIG. 1, two sets of communicating units are illustrated, each set functioning as an independent network. These networks are illustrated as networks 10 and 11 comprising a first set (set A) of nodes or units as well as a second set (set B) of nodes or units. Four units of the first set are shown as units 12, 14, 15 and 16. One unit of the second set is shown labeled 13. Each unit may be referred to as “terminal” or a “node”. Each unit 12, 13, 14, 15, 16 may be a fixed or portable data terminal, or a fixed or portable two-way radio, or indeed a video telephone or other communicating unit. The units 12, 13, 14, 15, 16 will simply be referred to hereafter as “radio units”. Each set of radio units consists of two or more radio units communicating with each other. While any member of one set may interfere with the transmissions of one or more members of the other set (and any further sets not illustrated), it is possible, indeed probable, that any given radio unit may not be able to directly receive the transmissions of such other radio units that it may interfere with or that may interfere with it, thereby making conventional methods for avoiding interference as listen-before-talk (carrier sense) ineffective.
Interference often hinders performance of communication systems. One type of interference often encountered by a user within a communication system is interference generated by the transmissions of other users. This is typically caused by many users transmitting within the same frequency band, and referred to as co-channel interference. In order to reduce co-channel interference many communication systems employ a frequency reuse pattern, where adjacent transmitters transmit on different frequencies. However, given the price of spectrum, future communications systems will be characterized by aggressive frequency reuse patterns that will result in significantly increased levels of co-channel interference.
In order to reduce co-channel interference, the “Listen-before-transmit (LBT)” etiquette has been formulated in the past in decentralized RF environments to enable non-interoperable systems to share spectrum. In such systems a node defers from transmitting if the received power is above some predefined threshold during a time period in which the node was to transmit. One assumption of a LBT system is that overlapping transmissions will almost certainly jam each other. This is approximately correct in the case of user devices simultaneously attempting to access a base station, but is definitely not true in peer-to-peer communications between disjoint pairs of nodes, where there is little correlation between the RF power detected by the node that intends to transmit on the channel and the interfering power impacting the intended receiver. The effect is that the would-be transmitters often unnecessarily refrain from transmitting some of the time and at other times may transmit when the intended receivers are jammed by interference.
This fact has been partly addressed in the proposed SAMA (Simple Asynchronous Multiple Access) etiquette discussed in U.S. Pat. No. 5,987,018, where the initial transmission called a Probe is sent to the intended receiver in a chosen time slot. If the receiver receives the Probe it will send an ACK (CTS—Clear-to-Send) to the transmitter indicating that the chosen time slot is acceptable. The reception of both the Probe and the ACK is the indication to the transmitter that it can continue using the slot it originally chose. It should be noted that the term slot does not imply any synchronization between radio nodes but is more a convenient term for a specific time interval inside the frame. The functioning of the SAMA etiquette is shown in FIG. 13.
As illustrated in FIG. 13, Node A sends a probe to Node B. If, Node B receives the probe, and if Note B determines it is a desired location for reception of Node A's transmission, then Node B will send a CTS message to Node A indicating that Node A should continue transmitting within that particular location. If the Probe or CTS is not successful in reaching its destination, a new Probe is sent at another point inside the frame and the process is repeated. The frame is a fixed time interval agreed by all participating SAMA nodes. SAMA etiquette purposely leaves it open to the designer's choice to decide when the Probe/CTS should be sent. One mentioned option is to use LBT. From these attempts, it was proposed that each node create a table of accessible and inaccessible slots. Even though this technique does improve upon LBT etiquette, it still presents a problem in that multiple attempts to communicate with a node may be made to build the table, and to determine an appropriate location for transmission. These multiple probes contribute to system interference. Therefore, a need exists for a method and apparatus for transmitting information within a communication system that improves upon the LBT etiquette, yet does not generate the interference caused by the existing SAMA etiquette.